This invention relates to a spectral-sensitized silver halide photographic emulsion and, more particularly, to a silver halide photographic emulsion made highly sensitive to white light, especially to red spectral region by the use of a combination of at least two sensitizing dyes.
The silver halide photosensitive materials are required to be highly sensitive to specific wavelength regions which vary with the purpose of use of particular photosensitive material. It is well known that as one of the techniques in producing such a photosensitive material, a certain type of sensitizing dye is added to the silver halide emulsion to increase effectively the sensitivity to the specific region of wavelengths longer than those characteristic of a silver halide. It is further known that when the said sensitizing dye is used in combination with a certain other type of sensitizing dye or a certain organic compound, the emulsion is imparted with a sensitivity greater than the sum of sensitivities imparted by the individual dye or compound. Such an enhanced effect is called supersensitization and a number of such combinations have been reported. There have recently been required more sensitive silver halide photosensitive emulsions and, hence, it is important to develop a technique capable of achieving more efficient spectral sensitization. In producing photosensitive materials of high sensitivity, it is most advantageous to use a combination of at least two sensitizing dyes which are in supersensitization relation and which accompany no desensitization. With the recent rapid progress in optoelectronics, laser and LED have come into practical use in place of conventional incandescent lamp and the like as the light source for converting a signal current into a light signal which is used in image processing. Especially, the emission wavelengths of He-Ne laser, ruby laser, and red light emission diode, which are already put into practical use, are in the spectral region of from 600 to 700 nm, but the actual situation is such that few of the conventional supersensitizing combination can impart sufficient red sensitivity to photosensitive materials which are to be used in recording red light within said wavelength region.